A separator for a lithium secondary battery is a porous thin film present between a positive electrode and a negative electrode, for facilitating the transmitting lithium cations during a charging/discharging process, and is generally produced with polyolefin resins such as polypropylene, polyethylene and the like in view of cost, chemical resistance, tensile strength, ionconductivity and the like.
The microporous separator made of a polyolefin resin is produced by a dry process using uniaxial orientation of an extruded polyolefin film, or a wet process which comprises blend/extrusion of liquid paraffin/high density polyethylene(HDPE)/ultra high molecular weight polyethylene(UHMWPE), biaxial orientation of the resulted product, and removal of liquid paraffin by using an organic solvent.
The wet process has some disadvantages such that it requires the use of liquid paraffin and an organic solvent, and the procedure is complex. In the dry process, particularly in the step of resin extrusion, the polymer chain is derived to be oriented to the machine direction(MD) so that crystallization can occur in the state that a lamellae layer is oriented to the transverse direction(TD), thereby directing the layered structure to be formed along MD. The dry process is simple and thus advantageous in terms of economy, as compared to the wet process, and eco-friendly as it does not employ an organic solvent.
As a safety function in a lithium secondary battery for preventing thermal runaway in the battery when it is overheated, a shutdown function is required in a separator. The term “shutdown function” refers to, when a battery is overheated, a function for preventing the battery from being further overheated by shutdown of the holes so as to block the migration of positive lithium ions before degradation of a positive electrode material occurs.
In the case of a monolayer film prepared via a dry process using a propylene homopolymer, an effective shutdown function cannot be exerted owing to the high melting point (about 160° C.) of the propylene homopolymer. On the contrary, in the case of a porous film prepared from a high density polyethylene, it is easy to ensure the shutdown properties since the high density polyethylene has a low melting point (about 130° C.). However, particularly in the case of a porous film prepared from a high density polyethylene via a dry process, mechanical properties such as tensile strength of thus obtained separator are inferior to a common polypropylene separator, and the battery can be excessively overheated, possibly leading to melt-down of the separator. For solving the problem, for example, U.S. Pat. No. 5,691,077 proposed a multilayer separator prepared by a dry process, which is formed of the layer of polypropylene/polyethylene/polypropylene. Such multilayer separator can advantageously have a shutdown function from polyethylene as well as high tensile strength/melt-down blocking function from polypropylene, however it can be obtained by far too complex manufacturing process which comprises preparing a polypropylene precursor film and a polyethylene precursor film respectively; and either carrying out lamination and elongation of the precursor films, or carrying out elongation of each precursor film, forming pores and laminating them. Therefore, owing to such complex process much higher manufacturing cost is needed and there is a risk of destruction of a pore structure during the lamination process.